What Is Vapor Barrier?

A vapor barrier is a low-permeability material that resists the movement of water vapor from one area to another. In residential construction, vapor barriers are most commonly installed in crawl spaces (on the ground surface), in below-grade walls, and sometimes between wall framing and interior finishes. Their purpose is to prevent ground moisture and outdoor humidity from migrating into the building structure where it can condense and support mold growth.

Crawlspace vapor barriers are the most common and impactful residential application. Without a vapor barrier, soil moisture evaporates continuously into the crawlspace air. In an unconditioned crawlspace, this moisture can reach wood framing, floor joists, and subfloor above — keeping them perpetually damp enough for mold and wood rot. A ground cover vapor barrier dramatically reduces this moisture load.

Installation standards have evolved significantly. Older guidance called for 6-mil polyethylene sheeting loosely placed on the crawlspace floor. Current best practice, particularly in performance building standards, calls for 12–20 mil reinforced polyethylene extended up the foundation walls and sealed to the wall with tape and mechanical fasteners. Seams should overlap by 12 inches or more and be taped. This "encapsulated crawlspace" approach nearly eliminates ground moisture migration.

Vapor barriers in wall assemblies are more nuanced. The appropriate vapor barrier strategy depends on climate zone: in heating-dominated climates, vapor barriers go on the warm interior side of insulation; in cooling-dominated climates, they may go on the exterior side; in mixed climates, "smart" vapor retarders that adjust permeability with humidity may be appropriate. Improper vapor barrier placement can trap moisture within a wall assembly and create worse mold conditions than no barrier at all.